1. Field of the Invention
This invention relates to a process for producing an organic polyisocyanate having a long storage life, and more particularly, to a process for producing an organic polyisocyanate having a long storage life which comprises reacting an aliphatic, alicyclic or araliphatic diisocyanate monomer with water in the presence of a specific solvent mixture.
2. Description of the Prior Art
Because of the presence of an isocyanate group which is reactive with an active hydrogen, organic polyisocyanates obtained by reacting an organic diisocyanate monomer with water find utility in preparing foaming agents, paints and adhesives. Of these applications, use of organic polyisocyanates as an ingredient of a paint is increasingly gaining acceptance because organic polyisocyanates provide a paint film with good physical properties.
A film of a paint prepared from aromatic polyisocyanates yellows, cracks or peels a few months after painting, whereas a film of a paint prepared from aliphatic, alicyclic or araliphatic polyisocyanates retains its initial physical properties for a few years without deterioration. Although aliphatic, alicyclic or araliphatic polyisocyanates have good physical properties, they also have inherent disadvantages. For example, reaction between an organic diisocyanate monomer and water inevitably produces polyurea as a precipitate (as disclosed in U.S. Pat. No. 3,124,605, U.S. Pat. No. 3,903,127, Japanese Patent Publication No. 16448/62, and Japanese Patent Application (OPI) No. 134629/74). According to the processes disclosed in these prior art references, a transient emulsion is formed due to the low miscibility of an organic diisocyanate monomer and water. The reaction between the diisocyanate monomer and water which is slightly dissolved in the diisocyanate monomer phase produces the desired polyisocyanate, but if water reacts with the diisocyante monomer slightly dissolved in the water phase, polyurea is precipitated as a by-product. Precipitation of polyurea is not only uneconomical but it reduces the commercial value of the polyisocyanate end-product.
The amount of by-product polyurea produced varies depending upon temperature, stirring rate, ratio of diisocyanate monomer to water, etc., but the amount is generally 0.5 wt% or more based on the weight of the polyisocyanate end-product. The production of polyurea as a by-product can be minimized by addition of 40 mol or more of diisocyanate monomer per mol of water, but it is impossible to completely prevent the production of polyurea as a by-product. Moreover, when this proportion of diisocyanate monomer to water is used, the large excess of diisocyanate monomer must be removed from the polyisocyanate, and this results in a complex manufacturing procedure, which is uneconomical.
U.S. Pat. No. 4,028,392 discloses an isocyanate prepolymer of a specific formula and a process for producing such. This process described in U.S. Pat. No. 4,028,392, comprises reacting a diisocyanate monomer with water in the presence of a hydrophilic organic solvent such as a carboxylic acid ester, a phosphoric acid ester, an amide, a ketone, a nitrile, an ether and the like. However, the disclosure set forth in U.S. Pat. No. 4,028,392 does not teach or suggest the use of the unique solvent mixture employed in this invention nor the advantageous results which are obtained when such a mixture is used over the use of the solvents individually, as demonstrated in Reference Examples 1-6 given hereinafter.
A further disadvantage with polyisocyantes produced using prior art processes is that the monomer content in the polyisocyanate increases on storage for a long period. This poses a serious problem in spray painting because any diisocyanate monomer contained in the mist of the paint can cause pollution problems, and can have an adverse effect on the human body as well as the painted object. Therefore, it is desired to minimize the diisocyanate monomer content in the polyisocyanate.
Commercially available polyisocyanates contain 1 to 2 wt% diisocyanate monomer and have a very high viscosity of about 6,000 to 10,000 cP at 30.degree. C., so that they must be diluted with a suitable solvent before use, and this has been another cause of pollution problems. It is therefore desired to be able to produce a high-solid type paint or a solventless paint using polyisocyanates of low viscosity. A polyisocyanate with a low viscosity can be prepared by increasing the molar ratio of the diisocyanate monomer to the water, but unfortunately this results in a further increase in the monomer content in the resulting polyisocyanate on storage for a long period of time.